Air laid (or dry laid) webs are presently produced by dispensing dry loose fibers, generally less than 1/4 inch long, from one or more distributors onto a moving foraminous forming wire to obtain a loose web having little strength or integrity. To give some strength to the loose web, the web is then consolidated between two compaction rolls or belts. The strength thus imparted enables transfer of the web from the forming wire to a foraminous carrier wire, which carries the web through subsequent bonding and drying operations. After drying, the web is wound-up on a parent roll. One advantage of dry-laid technology over more conventional wet laid processing is the increase in the bulk of the product web, which decreases the amount of fiber required per unit volume while improving softness. Generally, then, it is desirable to increase bulk if other required properties can be at least retained. In some end use applications improved bulk and softness are more important parameters than other properties, which can be reduced appropriately. Bulk as used herein is defined as the four ply caliper of the product web by a standard caliper gauge such as the TMI micronmeter divided by its basis weight increased fourfold.
Loose fibers used in dry-laid processes are typically prepared by defiberizing pulp rolls, laps or bales in a hammermill or its equivalent, said fibers then being transported to the distributor pneumatically. During defiberizing, the fibers are dried unintentionally to a moisture content of less than 3% by weight, generally less than 2%. At this moisture level, the fibers transported to the distributor are subject to electrostatic charge forces which interfere with proper fiber transportation and web formation in that they clump together or adhere to machine surfaces, and are particularly dangerous because of the potential for explosion. To overcome this condition, existing practice is to enclose the forming section of the web manufacturing process within a room of regulated temperature and humidity. With the room temperature typically at 70.degree. F. and at a relative humidity of about 70%, sufficient driving force exists for said fibers to absorb moisture from the humidified air. Through the length of the forming wire, and during the residence time of said fibers in the distributor and on the wire, usually between 4 and 15 seconds, the fibers will pick up 2 to 5% additional moisture before reaching the aforesaid consolidation rolls. Applicants have found that this increase in fiber moisture content deleteriously affects the bulk of the final web product. Applicants have found further that variations in bulk of the product web in the cross machine direction are attributable in part to the non-linearity of moisture content profiles transverse of the forming wire prior to consolidation.